Due to primarily federal regulations, engine manufacturers are being forced to reduce the amount of harmful compounds in the combustion exhaust. To effectively reduce the NOx concentrations in the exhaust stream of lean burning engines--including diesel and certain spark ignited engines--the present technology of NOx catalysts requires a sufficient concentration of hydrocarbon (HC) species to be present. In other words, in combustion exhaust purification systems having an oxygen environment above 3% concentration, some type of reducing agent, usually a hydrocarbon compound, must be introduced into the exhaust in order to achieve acceptable reduction levels of NOx compounds. Different means of adding the HC into the exhaust stream have been developed including the addition of HC injectors to continuously or periodically inject HC into the exhaust stream. Unfortunately, these methods of introduction of HC into the exhaust increase system complexity and cost and do not allow for optimum atomization and vaporization of the HC.
What is needed is an exhaust gas purification system that is easily controlled, performs effectively across a complete operating range of the engine, and achieves satisfactory reduction of NOx compounds in the exhaust while reducing the overall system complexity and eliminating the need for additional HC injectors.